Image co-registration is the process of determining a geometric transformation that needs to be applied to one (or more) images of a plurality of images of the same thing so as to bring them into register, i.e. so that the image features are located in the same place within the field of view on all the images. For a rigid registration, the transformation may involve translation and/or rotation, and may thus have up to 3 or 6 degrees of freedom for two- and three-dimensional image pairs respectively. A more general transformation may allow scaling and shear as well as translation and rotation, thus allowing 12 degrees of freedom in three dimensions, or may be more complicated still. In a medical context the image features will usually be anatomical features visible in the respective images. The images can then be combined to produce a composite image containing detail from both (or all) the individual images.
Two categories of image registration exist; landmark based registration and intensity based registration. With landmark based methods, the user either manually selects landmarks (e.g. artificial fiducials or anatomical landmarks) in both images, or landmarks are automatically detected, and the two sets of landmarks are then registered. Intensity based registration, on the other hand, uses a distance measure which is directly determined based on the intensities of the two images, to measure the level of agreement between the two images given a tentative registration transformation. Registration using Mutual Information is a common example of intensity based registration. Image registration is typically carried out by an automated iterative process which seeks to minimise the differences between the images. The quality of the result is then assessed visually by a suitably experienced operator.
Co-registration of medical images allows direct comparison between the images, fusion of the images, or the use of non-stereotactic images for stereotactic purposes. Most treatment planning applications (such as for radiotherapy applications) have support for image co-registration.